Conventional refrigeration technology typically utilizes a heat pump that relies on compression and expansion of a fluid refrigerant to receive and reject heat in a cyclic manner so as to effect a desired temperature change or i.e. transfer heat energy from one location to another. This cycle can be used to provide e.g., for the receiving of heat from a refrigeration compartment and the rejecting of such heat to the environment or a location that is external to the compartment. Other applications include air conditioning of residential or commercial structures. A variety of different fluid refrigerants have been developed that can be used with a heat pump in such systems.
Certain challenges exist with these conventional heat pump systems. While improvements have been made, at best heat pump systems that rely on the compression of fluid refrigerant can still only operate at about 45 percent or less of the maximum theoretical Carnot cycle efficiency. Also, some fluid refrigerants have been discontinued due to environmental concerns. The range of ambient temperatures over which certain such refrigerant-based systems can operate may be impractical for certain locations. Other challenges with heat pumps that use a fluid refrigerant exist as well.
Magneto caloric materials (MCMs)—i.e. materials that exhibit the magneto caloric effect—provide a potential alternative to fluid refrigerants for heat pump applications. In general, the magnetic moments of a normal MCM will become more ordered under an increasing, externally applied magnetic field and cause the MCM to generate heat. Conversely, decreasing the externally applied magnetic field will allow the magnetic moments of the MCM to become more disordered and allow the MCM to absorb heat. Some MCMs exhibit the opposite behavior—i.e. generating heat when a magnetic field is removed and becoming cooler when placed into the magnetic field. This latter type can be referred to as inverse or para-magneto caloric material. Both normal and inverse MCM are referred to collectively herein as magneto caloric material or MCM. The theoretical Carnot cycle efficiency of a refrigeration cycle based on an MCM can be significantly higher than for a comparable refrigeration cycle based on a fluid refrigerant. As such, a heat pump system that can effectively use an MCM would be useful.
Challenges exist to the practical and cost competitive use of an MCM, however. In addition to the development of suitable MCMs, equipment that can attractively utilize an MCM is still needed. In certain constructions, one or magnets are used to create the magnetic field that causes the MCM to exhibit the magneto caloric effect. Although necessary, such magnets contribute substantially to the overall costs of a heat pump that uses an MCM.
Additionally, as stated above, the ambient conditions under which a heat pump may be needed can vary substantially. For example, for a refrigerator appliance placed in a garage or located in a non-air conditioned space, ambient temperatures can range from below freezing to over 90° F. Some MCMs are capable of accepting and generating heat only within a much narrower temperature range than presented by such ambient conditions. Also, different MCMs may exhibit the magneto caloric effect more prominently at different temperatures.
As further described below, one approach for providing the cooling needed in e.g., refrigerator applications can be to use multiple different MCMs having different response temperatures in a manner that provides the overall temperature change needed. However, as the contents of e.g., the refrigerator are lowered in temperature, subjecting all of the MCM to the same magnetic field strength can be unnecessary and inefficient.
Accordingly, a heat pump system that can address certain challenges such as those identified above would be useful. Such a heat pump system that can also be used in e.g., a refrigerator appliance would also be useful.